The present invention relates to an elastic element for use in a bone anchoring element, a connecting element, a rod, and a stabilization device and a method for manufacturing the same.
It is known to use fixation and stabilization devices to fix fractures and stabilize spinal columns. These fixation and stabilization devices commonly comprise at least two bone anchoring elements or bone screws. Each of the bone anchoring elements is anchored in a bone or vertebra and is connected by a rigid plate or a rod. These types of fixation and stabilization devices generally do not allow any movement of the bones or vertebrae relative to each other.
In some instances, however, it is desirable to stabilize the bones or vertebrae so that the bones or vertebrae can carry out limited, controlled motion relative to each other. This is known as dynamic stabilization. Dynamic stabilization devices commonly comprise an elastic element instead of a rigid plate or rod that connects each of the bone anchoring elements.
One example of a dynamic stabilization device for vertebra is disclosed in United States Patent Application Publication No. 2003/0109880 A1. The dynamic stabilization device comprises first and second screws that are each anchored in a vertebra. Each of the screws has a receiving member for insertion of a spring which thereby connects the screws. The spring is provided in the form of a helical spring having closely neighboring coils like a tension spring. The spring is fixed in the receiving members by clamping screws. In this arrangement, however, because the spring is flexible, the spring can evade the pressure of the clamping screw and therefore become unfixed from the bone screw. Furthermore, both the elasticity and the flexural strength of the spring depend on the length of the spring. Thus, in applications requiring a spring with a short length, the elasticity and flexural strength of the spring is relatively small.
Another example of a dynamic stabilization device for a joint such as a wrist or knee joint is disclosed in U.S. Pat. No. 6,162,223. The dynamic stabilization device comprises a rod having a proximal rod section and a distal rod section connected to bone pins. The proximal rod section and the distal rod section are connected to each other by a flexible spring. The proximal rod section, the distal rod section, and the flexible spring are arranged outside of the body. The proximal rod section and the distal rod section are not fixedly connected to the flexible spring, but can move freely along a bore therein. In this arrangement, the flexible spring must be formed to have a diameter larger than a diameter of the rod. Additionally, the flexible spring must be large in order to have a high flexural strength. This dynamic stabilization device therefore has a complicated and voluminous structure, which prevents the dynamic stabilization device from being used inside the body on spinal columns.